Radio receiving apparatus



RESPOA/f "aspen/s:

Jan. 31, 1933, 1,895,792

A. H. GREBE RADIO RECEIVING APPARATUS Filed Dec. 24 1930 2 Sheets-Sheet 1 17E SPOA/SE 5'00 400/0 Fif'QUE/VC) if S Pall/.5 E

f 2 B vllrjvzmoR Jan. 31, 1933. A. H. GREBE RADIO RECEIVING APPARATUS Filed Dec. 24, 1930 2 Sheets-Sheet 2 RE ui/ve' r igQ/VENT R I BY ZTTW Patented Jan. 31, 1933 UNITED STATES PATENT ol-"F ler;

ALFRED n. GREBE, or HOLLIS, :uEw YORK RADIO ancmvme APPARATUS Application filed December 24,1930. Serial No. 504,566.

My invention deals with what is probably the most important characteristic of the modern broadcast receiver, the fidelity with which the radio set reproducesthe programs of a transmitting'station. 1

It is comparatively easyto make a radio.

receiver sensitive, and selective; these also are two important;characteristics of a good radio frequency amplifier; of course, there aremany other parts such as antenna, power supply, loud speaker, etc., but I shall consider only the first three. A

The radio frequency amplifier takes the signal from the antenna, or other pick-up device, and amplifies it several thousand times, beforesupplying it to the detector. Now, a broadcast signal is made up of a whole band of frequencies and not a single frequency. Thus, if'the carrier frequency of the station is 1,000,000 cycles, and a musical program is being broadcast, the stationis sending out practically all frequencies between"'1,005,000 cycles and 995,000 cycles. V

That is, a Whole band of frequencies-4n this case 10,000cycles wide, is radiated from the transmitting antenna and picked up by" the receiving antenna.

All of these frequencies should be amplified two different signal frequencies;'they are the so-called resonance curves 'of the' set. There is'also shown in'Fig. 1', thefrequency width of a broadcast signal band over which the radio frequency amplifier should amplify uniformly, namely, 10,000 cycles. It will be seen that for neither signal does the circuit amplify uniformly all of the frequenciesinvolved in the signal, and thatthe' difference in amplification of'the various components of the signal is much greater, for the lower frequency f than it isfor The ffnon-uniformity or cast signal'is a defect in the ordinaryflradio frequency amplifier which is overcome only g I amplification throughout the "frequency band of a broadat the expense of complexity of circuitarrangement and operations,andconsequently of the cost of construction WVhena set acts to give an amplification whichvaries'appreciablyf throughout a band of frequencies sufiiclently wide to reproduce well"an-ordi-' nary broadcast program, it may be said to have an amplification; response sensibly nonuniform throughout the, frequency band of abroadcast signal.

The detector of the modern receiver changes the radio frequency signal intoone of audio frequency; after passing-through the detector the signalconsists only of cur-'- rents with frequenciesoffrom-say150't0 5,000 cycles per second; vThiscurrent is then sent through an'audio frequency amplifier and here also a non-uniform amplification is ob curve for. anaudio-frequency amplifier showing best amplification in'the middle of the frequency range-and much'lower amplificationat both high and low frequencies.

tained. In Fig; 2 i's'shown'normal response This response curve isdrawn'on the assump tion that all of the frequenciesare supplied to the audio frequency amplifier with the same amplitude but I now consider that the radio frequency amplifier has amplified the low notes of the signal more than the high notes and so the response of the amplifier, with this defective signal su plied to is shown'in Fig. 3. Indotte'd linesfl the curve of Fig. 2 is duplicated and in full linesi's shown the response of the-audio frequency amplifier to the output ofthe detector.

This curve shows an even faster falling off of the high frequency components of the signal thanthe audio frequency amplifier itself would give. This then is the response curve of the average radio receiver, which has no corrective measures applied.

My invention consists'of a proper combinatlon of corrective factors operating onthe audio frequency amplifier to make the output oft-he receiver practically the same over the whole range of useful frequencies.

By utilizing the phenomenon of resonance in the secondary circuit of one of the audiofrequencytransformers, the response curve can be changed from'that' of Fig. 3 to the solid line curve of Fig. 4. Here .the responseof the audio frequency'amplifier has been great- 1y increased in the frequency band just below 5,000 cycles and has actually been made greater at 4,500 cycles than itis'at 500 cycles. Further ithe cut off frequency, beyond which the audio frequency amplifier gives an appreciable response, has been made much more definite, and lowered from what it was 2 This feature of the audio fre- V quency amplifier results in much less tend 7 over av larger frequency range.

ency for interference between broadcasting stations operating in neighboring channels.

' r The hump in the response curve, sh own at I. A in Fig. v4:, can be accentuated o'r diminished by-making the resonance phenomenon 1n the be lessened, and at the same time, spread'out To bring up the response of he audio frequency amplifier inthe low frequency range, '-other means must be employed, for it can be seen that the response curves of Figs. 3 andff coincide in, thelower frequency range. I have I found that the same means, properly applied,

will give both of thecorre'ctions desired.

,It has been suggested that a'very great inductance be used in the primary of the' audio frequency amplifier transformer but when such is attempted, difliculty is encountered in retaining the proper ratio of turns without at the same time losing the resonance response phenomenon analyzed above. I have found that by using a'condenser, of proper capacity, in series with ,an adjustable resistance of proper range, I can alter the response, curve of Fig. 4,.to thatlof Fig.5. In the latter figure,the resp'onsecurve of Fig.4 is reproduced in'the dottedcurve and in the full line curve is shown the'equa-lized response tov be obtained -when using this additional corrective factor. J In Fig. 6, I- have shown a ,radio receiver the uncorrected response.

consisting of the radio frequency amplifier indicated by the letters RR A, detector D, and the output of the detector is shown supplying the input circuit of a push pull am-' plifier indicated at T, which, in turn, supplies power to the loud speaker, not shown.

The low frequency corrective effect comes from thisaction of the circuit.C-R, shunting the primary winding of the audio frequency transformer, and the high frequency corrective effect comes from the'action of the circuits G R and C R shunting the secondary' windings of the transformer T, in

which, by mutualinduction between the two transformer windings, has .a marked effect in the action of the secondarycircuit. 4 i V The constants ofthese corrective circuits applied to both primary and secondary windings of the transformer T, and'the amount of addition to theeffect of the CR circuit,

mutual inductance between the windings,

must be of suitable-magnitude and relative proportions, *to gain thebeneficial effectswhich make for the difference in the response curves of Fig.5 and Fig. 3, the latter being I have found that transformers havingone mil of unimpregnated paper jdielectric. be-

tween layers in the secondary winding and having about 500 henriesof inductance in this winding of #4011 80 S'.-enamel wire, give about the proper internal capacity andeflem tive resistance to serve for C C and B R of Fig. 6. i

In combination with these values of the secondary corrective circuits," I have-found that if the primary -of the transformer has 7 about fifty henriesof inductance, .a reason;

able value of capacity C is .05 microfarad and for a. properly cooperating resistance, R should be var iable from 2,000 to 100,000

ohms.

I have foundthat avalue of R, which gives 0 I the proper low frequency correction ,forj a radio signal at one limit ofthe broadcast frequency range is not-suitable for asignal' at the other end ofthe'broadcast spectrum.- In

fact, for best results R should be continuously I i quency of the signal 'beingreceivedr The reason for this requirement undoubtedly lies in the difference intheresponse 'curves of the radio frequency amplifier Vase-illustrated in 1, and analyzed in connect'ion with the discussion of this figure.

By using my inventive idea above outline d,

variable and adjusted according tiowthe-frei it is possible to getan essentially uniform 7 response curve over the whole: of the range of useful audio frequencies- Whereas the same result may possibly be attainedby other means, these are much more complicated and costly than the apparatuslguse. .j

In Figure 7 I have shown in curve the fidelity curve ofa; modern radio receiver, as it comes from the manufacturer 1 and in the two other curves, Band C, I have shown the response curves of the same receiver, when it is equipped with my corrective circuit, both when this is properly and improperly adjusted. I 7 I In curve B, insuiiicient resistance was used in the corrective circuit so that too much correction was applied in. the low frequency range and the defect in the high frequency range was actually augmented. A proper value of resistance in the corrective circuit, however, resulted in the performance given by curve C, which is almost 100 per cent perfect response, in the frequency range from 60 to 5,000 cycles Having thus described my invention, what I claim is:

1. In a radio receiving set an audio frequency amplifier having a sensibly non-uniform response throughout the band of useful audio frequencies, an audio frequency transformer circuit in which there is the combination of 'a secondary circuit effectively resonant to a frequency near the upper end of the audio frequency range and a primary circuit reasonant to a frequency near the lower end of the audio frequency range.

2. In a radio receiving set an audio frequency amplifier having a sensibly non-uniform response throughout the band of useful audio frequencies, corrective circuit arrangements which result in the secondary circuit of a transformer of the audio frequency amplifier being resonant for frequencies at the. upper limit of the audio range and the'pri-.

mary circuit of the transformer being resonant for frequencies at the lower limit of the audio range.

ful audio frequencies,- a. frequency corrective arrangement consisting of an audio frequency transformer, with a suitable condenser arranged to have two resonant frequencies, one

which lies near the upper limit of the useful audio frequencies and the other near the lower 3. In a radio receiving set an audio frerangement consisting of an audio frequency transformer and a suitable condenser, 'arsistancebeing connected in parallel withthe primary of one of the transformers ofthe' audiofrequency amplifier, said condenser being of such capacity as to produce, in con junction with the'prim'ary winding of said transformer, parallel resonance for a frequency substantially at the lower limit of the audio range. s

5. In a radio receiving set an audiofrequency amplifier having a sensibly non-uniform response throughout the band of use- 

